The present invention is related to the field of robotics, in particular to the field of robot assisted surgery. The invention is particularly related to manipulators for minimal invasive surgery (MIS) where instruments are to operate precisely inside the human body while only being granted limited access through small entry incisions and where low volume occupancy above the operation table is considered highly desirable.
In MIS, instruments can only reach the region of interest through the entry point into the body typically foreseen of a trocar. The trocar constraints the access to the region of interest, leaving only 4 degrees of freedom (DOF), compared to 6 DOF in open surgery, in the case rigid instruments are used. FIG. 1 shows the available degrees of freedom DOF1, DOF2, DOF3 and DOF4 consisting of three rotational DOFs (DOF1, DOF2 and DOF4) and one translational DOF (DOF3). The entry point is a pivotal point which causes motion reversal. The instrument tip moves in a direction opposite to the motion of the surgeon's hand. Further, the leverage of motion changes with increasing insertion depth. This makes accurate positioning more difficult at greater depth. The incision points do not provide stable pivots either. In order to achieve accurate instrument positioning, manipulators are used which physically constrain the instrument to pivot around a fixed point that coincides with the incision.
Several remote center of motion (RCM) mechanisms that orient an end-effector about two intersecting axes at a fixed geometric location in space have been developed in the past for use as manipulators in MIS, such as the ones described in U.S. Pat. Nos. 5,397,323, 5,817,084 and WO 2004/037103. When the RCM is aligned properly with the entry port into the body, the instrument can only pivot around the RCM point and is thus physically constrained and not able to exert large forces upon the body wall. Alternative systems e.g. applied to conventional serial or parallel robot mechanisms rely on software constraints to make the instrument pivot around the entry-point of the body. Such approach is often described as a ‘virtual’ or ‘soft’ RCM. Motion about an RCM is achieved by coordinated motions of multiple joints, many of which may be required to make fairly large motions in order to achieve relatively small tool reorientations. Such virtual RCM systems can be bulky and need to be sufficiently powerful to ensure good dynamic behaviour at the instrument. In case of failure it becomes difficult to guarantee patient safety, which makes them less suitable for MIS.
Apart from safety, sterilisability and reachable workspace inside the patient, accessibility and achievable precision form other crucial properties of instrument manipulators.
Since the majority of MIS interventions are conducted with multiple simultaneously handled instruments, it is necessary that instrument manipulators are compact and occupy as little as possible space above the patient. This is essential for finding configurations of instrument holders with non-overlapping workspace that can tackle various multi-instrument tasks. Also for cases where the workspace is shared with the surgeon, e.g. when some of the instruments are managed by the latter, the instrument manipulator should take in as few as possible not to hinder the surgeon. Furthermore, the space taken up by the instrument manipulator should be well-defined and predictable so that there is little room for surprises (surprising robot movements) and the surgeon can easily avoid collisions with the robot.
Traditional mechanisms that employ a combination of linkages and parallelograms to achieve a ‘mechanical’ RCM score better in terms of affordability and compactness. However, current mechanisms typically only support remote actuation of the distal two rotational degrees of freedom DOF1, DOF2. Such solutions make use of a translation stage located at the end-effector to provide the linear translation of the instrument along its axis through the insertion point, inwards and out-wards into the patient's body (translational degree of freedom DOF3). Often additional means to rotate the instrument about its axis (rotation degree of freedom DOF4) are additionally mounted on top of such stage. Such translation/rotation stages not only take in a lot of space above the patient, blocking the access and view of the surgeon or visualisation devices; they also present a relatively large mass at the robot's end-effector limiting the dynamic range. The stage and actuators form a variable load upon the lower elements of the kinematic chain; they jeopardize the achievable positioning accuracy, complicate the design of gravity compensation methods and further affect patient safety as actuators move in close vicinity to the patient. With stages it also becomes more difficult to guarantee safety, maintain sterility and so on.
Taylor (U.S. Pat. No. 5,397,323) describes an RCM-mechanism formed by a double set of parallelograms which features local actuation of the remote translational degree of freedom DOF3 (U.S. Pat. No. 5,397,323, FIG. 1D). The proposed approach relies on a pair of telescopic arms that form two opposite links of the first parallelogram at the mechanism's base. By extending or shortening both telescopic links in equal amounts, the parallelogram can be deformed into a larger or smaller parallelogram. As a result the instrument which is parallel to the respective telescopic arms will be displaced along its own axis passing through the remote center of motion and as such creating a translational degree of freedom DOF3.
For such method to work it is imperative that the telescopic arms move precisely and at all times in equal amounts, otherwise the RCM is lost. This can be achieved by employing two linear actuators that are controlled to move in a synchronous fashion at the cost of additional complexity, inertia and reduced reliability. An alternative exists in simply adding an extra connection bar between the telescopic arms to form an additional parallelogram over these arms. This parallelogram will carry the non-actuated telescopic arm along with the actuated one. However, when the angle between the parallelogram's links becomes 90 degrees, this parallelogram might transform and shift towards an isosceles prism configuration, in which case the RCM is not maintained either.
Methods to circumvent this safety problem are complex, bulky and/or affect the achievable positioning precision of the mechanism. For example, a possible solution presented in U.S. Pat. No. 5,397,323 FIG. 5 exists out of an additional mechanism that consists of a set of 5 bars and 6 additional pivot points. For the mechanism to work the lengths of 4 out of 5 bars must be exactly the same, the length of the fifth bar must perfectly match the length of the side of the main parallelogram. If this is not the case the mobility of the mechanism can be completely lost and the parallelogram will be unable to extend or retract. When adding play to the pivot points to relax these tight manufacturing constraints, the stability of the RCM point and the precision of instrument positioning suffers.
A second possible solution presented in U.S. Pat. No. 5,397,323, FIG. 6, provides in a pair of pulleys attached at the extremities of a common rigid bar, connected through a belt-like mechanisms. The bar and pulleys move over two rolling surfaces that slide along a pair of linear bearings. Two assumptions must be fulfilled for this system to work. Firstly, both pulleys are to roll without any slip over the rolling surfaces. If one relies on rolling friction and pre-tensions hereto the components to achieve sufficiently high friction, pre-tensioning should be designed for worst-case loading of the mechanism to prevent slip from occurring under all possible circumstances. Such methods will introduce additional friction into the mechanism, typically limiting the smoothness of motion of the mechanism and will require a substantial torque/force to get the system running. Depending on the friction it becomes difficult to allow small incremental motion. Also, if at some point slip did occur it is not straightforward to detect and rectify this. Alternative methods that would rely e.g. on rack-and-pinions or timing belts, must be designed to be free of play. Zero-backlash versions require precise manufacturing, are costly and add substantial amounts of friction. Secondly, the two pulleys must rotate at all times at exactly the same speed. This would again require high pre-tensioning of very stiff belts introducing additional friction, or the use of precision timing belts free of play.
While the abovementioned approaches aim to avoid the loss of the RCM caused by a shift from the corresponding parallelogram to an isosceles trapezoid, they may at the same time further reduce the achievable workspace of the mechanism due to internal collisions between the additional plurality of links.
In fact the workspace by the proposed solution is somewhat restricted even without above corrective means. Since the instrument axis and the telescopic arms from the driving parallelograms are always parallel for acute and/or obtuse instrument angles also the angles of the driving parallelograms will be acute and/or obtuse. Internal collisions between these parallelograms will prevent reaching such acute and/or obtuse orientations.
The same parallelism property also allows for limited room in optimizing dynamics, manipulability, gravity compensation mechanisms and the like. For example regarding gravity compensation the parallelism requires to compensate for often coinciding worst-case loadings at instrument and driving mechanisms. These compensation means are therefore typically larger than in cases where instrument axis and driving mechanism can have a different orientation.
Further, the transmission from the local to the remote site of the mechanism relies on two parallel connecting bars that move relatively with respect to each other. Therefore the overall connection between local and remote site is rather bulky. It becomes also difficult to realize longer distances between local and remote site as this requires the manufacturing of a pair of long, possibly complex, bars under tight manufacturing tolerances where small variations in manufacturing will cause the loss of mobility or might require introduction of play and subsequent loss of accuracy.
Another system that departs from the double-parallelogram approach as introduced by U.S. Pat. No. 5,397,323 is described in WO 2004/037103. Whereas U.S. Pat. No. 5,397,323 employs two sliding links and means to make both links move at equal amounts in order to create the translational degree of freedom DOF3, WO 2004/037103 introduces a method that employs as much as four sliding links to establish two planar degrees of freedom DOF2, DOF3. Also, this approach is based upon the double parallelogram mechanism, whereby the first parallelogram at the base is being shortened and stretched in the longitudinal direction parallel to the instrument axis.
WO 2004/037103 foresees in two sets of double parallelogram mechanisms that operate in parallel planes. The individual parallelograms are connected via horizontal connection bars and jointly move when actuated. Four slides are thus needed to shorten and stretch the pair of first parallelograms closest to the base. The lower parallelograms are organised in two pairs of two links that pivot and slide at a constant offset around two parallel axes. The angle between sides of the parallelogram which corresponds to DOF2 is controlled by a motor parallel to the pivot axis and transmitted via belt mechanism. Through the various connection bars between the parallelograms this motion is transmitted to the different sliding bars and finally towards the instrument. WO 2004/037103 foresees in a pair of rack and pinions to provide translational degree of freedom DOF3.
A belt is used to synchronise the motion of the pair of pinions. Again, imperfect synchronisation leads to loss of the RCM. As described above, relying on high pre-tensioned belts is not reliable as correct synchronisation cannot be guaranteed and large amounts of friction are introduced. In such case costly high-precision zero-play timing belt, pinions and high-precision zero-play rack and pinions need to be employed. All components must be stiff to ensure correct synchronisation under varying load. As a result the entire assembly becomes heavy and quickly cumbersome in assembly.
Similar to U.S. Pat. No. 5,397,323, the method by WO 2004/037103 requires instrument axis and driving axes to be parallel. Limitations on workspace, limited flexibility in designing dynamics, manipulability, gravity compensation means identified in U.S. Pat. No. 5,397,323 are also present in WO 2004/037103.
Also, the transmission from the local to the remote site of the mechanism further relies on four parallel bars that move relatively with respect to each other, this complicates the design of a compact end-effector that needs to accommodate for these four connecting bars.
To progress MIS practice it would be desirable to devise and implement mechanical solutions that are not or to much less amount hampered by abovementioned drawbacks. In particular, the systems described above with remotely actuated translational degree of freedom DOF3 do possess a large range in the mechanism's roll angle of rotation DOF1, but only allow a limited working range in the pitch rotation angle DOF2 and require additional means to overcome straight angles where parallelograms might otherwise shift into isosceles trapezoid resulting in a loss of the RCM.